physics 2111 unit 6static friction the ramp questions in the pre lecture. i found dealing with...
TRANSCRIPT
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Physics 2111
Unit 6
Today’s Concepts:
Friction• Static Friction
• Kinetic Friction
Centripetal Forces
Mechanics Lecture 6, Slide 1
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Where are we?
Mechanics Lecture 3, Slide 2
Still doing examples of
different specific forces
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Stuff you asked about:
We got this guy
half atwoods are annoying, would like some focus there
These all seem to come off as "trick question" and the only way to get the correct answer is a free body diagram. It seems that my mental intuition is wrong quite often while my free body diagrams always lead to the correct answer. These question seem relatively easy but just take time to set everything up and have the answer appear.
Static and Kinetic frictions formulas. How they would apply to a problem with the previous concepts we know
can we go back to getting feedback right away? If I'm wrong, I like to know so i can find out where I'm making mistakes
Static friction
The ramp questions in the pre lecture.
I found dealing with friction in circular motion to be confusing. How exactly is the formula derived?
More examples on static friction.
I found the equations with trig to be a bit confusion
Friction is easy to understand, but it gets complicated to understand when you throw in mg, ma, and the natural force.
It is cool to think that an object can accelerate due to kinetic friction (ex: a box on a moving pickup truck). Mechanics Lecture 6, Slide 3
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Properties of Friction
Two types of friction:
Static friction (surfaces don’t move with respect to each other) - has maximum value:
Kinetic Friction (surfaces move with
respect to each other) – has constant
value:
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Friction
Always opposes the relative motion of two surfaces
Mechanics Lecture 6, Slide 5
Fgrav
Framp
Ffric
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Example 6.1 (Friction)
A 20kg block slides across a horizontal concrete floor. The co-efficient of kinetic friction, mk, between the floor and the block is 0.2.
What is the force of friction on the block?
Mechanics Lecture 7, Slide 6
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Question
The 20kg block now slides up a ramp that is inclined at 30o. The co-efficient of kinetic friction, mk, between the ramp and the block is 0.2.
What can we say about the force of the ramp on the block (the “normal” force)?
Mechanics Lecture 7, Slide 7
30o
A) |𝐹| < 196N.
B) |𝐹| = 196N
C) |𝐹| > 196N
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Example 6.2 (Friction on a ramp)
The 20kg block now slides up a ramp that is inclined at 30o. The co-efficient of kinetic friction, mk, between the ramp and the block is 0.2.
What is the force of friction the block?
Mechanics Lecture 7, Slide 8
30o
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Prelecture Question
Unit 6, Slide 9
In both cases shown a box is sliding
across a floor with the same kinetic
coefficient of friction and the same initial
velocity. The only difference between the
two cases is the mass of the box.
A. Case 1
B. Case 2
C. They will be the same
In which case will the box slide the furthest
before coming to rest?
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Checkpoint:
A block slides on a table pulled by a string attached to a hanging weight. In Case 1 the block slides without friction and in Case 2 there is kinetic friction between the sliding block and the table.
In which case is the tension in the string biggest?A) Case 1 B) Case 2 C) Same
Unit 6, Slide 10
Case 2(With Friction)
Case 1(No Friction)
m2
m1
g
m2
m1
g
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What we thought…..
In which case is the tension in the string biggest?
A) Case 1 B) Case 2 C) Same
A) when there is friction acting on something it makes it harder to move the object so it would require more tension force to move the box
B) M1 will not be accelerating as fast in case two than in case one because there is the extra force of friction acting against mass 1. Since the acceleration is smaller in case two, there has to be more of a force acting against gravity, the only other possible force is Tension..
C) The tension would be the same because the masses of the blocks remain the
same, so nothing will change the force of tension. Friction will only cause the
second case to accelerate more slowly..
Mechanics Lecture 6, Slide 11
Case 2(With Friction)
Case 1(No Friction)
m2
m1
g
m2
m1
g
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In which case is the tension in the string biggest?A) Case 1 B) Case 2 C) Same
Some popular wrong explanations for picking B) :• Without friction there is no tension.
• Because in case 2 the boxes are accelerating
• in case 2, M2 does not move, thus the string will be taught and will have more tension than in case 1
Mechanics Lecture 6, Slide 12
Case 2(With Friction)
Case 1(No Friction)
m2
m1
g
m2
m1
g
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Example 6.3 (Two blocks with friction)
A block (m2 =10kg) slides on a table pulled by a string attached to a mass (m1=5kg) hanging over the side. The coefficient of kinetic friction between the sliding block and the table is mk=0.2. What is the acceleration of the block?
Mechanics Lecture 6, Slide 13
m2
m1
g
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Question
A block (m2) slides on a table pulled by a string attached to a mass (m1) hanging over the side. The coefficient of kinetic friction between the sliding block and the table is mk.
What would be a good limiting case check in this situation?
Mechanics Lecture 6, Slide 14
m2
m1
gA) as uk gets bigger |a| should increase
B) as uk gets bigger |a| should decrease
C) as uk gets bigger g should go to zero
D) as uk gets bigger g should go infinity
E) as uk gets bigger |a| should not be affected
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Question
A block (m2) slides on a table pulled by a string attached to a mass (m1) hanging over the side. The coefficient of kinetic friction between the sliding block and the table is mk.
What would be a good limiting case check in this situation?
Mechanics Lecture 6, Slide 15
m2
m1
gA) m1>>>m2, a goes to zero
B) m1 >>>m2, a gets bigger
C) m1>>>m2, a goes to infinity
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Example 6.3 (Two blocks with friction)
A block (m2 =10kg) slides on a table pulled by a string attached to a mass (m1=5kg) hanging over the side. The coefficient of kinetic friction between the sliding block and the table is mk=0.2. What is the tension in the string?
Mechanics Lecture 6, Slide 16
m2
m1
g
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A force, F, pushes four blocks
across a frictionless surface. All
blocks accelerate at the same
rate Which of the below best
represents the freebody diagram
(FBD) of block 3?
(A)(B) (C) (D)
Question
1
3
4
2
3 33 3
Mechanics Lecture 6, Slide 17
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A force, F, pushes four blocks
across a frictionless surface.
Question
1
3
4
2
If I wanted to draw a FBD in which the only
horizontal force was the frictional force from block
2 onto block 3, what would I draw the FBD of?
A) block 3 B) block 2
C) blocks 2 and 3 D) blocks 3 and 4
E) blocks 1 and 2
Mechanics Lecture 6, Slide 18
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In the diagram to the right:
- the normal force of 4 on 3 is 40N,
- the weight of 3 is 50N
- the frictional force of 4 on 3 is 30N.
Question
3
What is the total force of block 4 on block 3?
A) 50N B) 70N
C) 90N D) 100N
E) 120N
F43
FFRIC-43
FGRAV-3F23
FFRIC-23
Mechanics Lecture 6, Slide 19
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Static Friction
Static friction (surfaces don’t move with respect to each other) - has maximum value:
Kinetic Friction (surfaces move with
respect to each other) – has constant
value: ms>mk
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Checkpoint: Question
Mechanics Lecture 6, Slide 21
A box sits on the horizontal bed
of a moving truck. Static friction
between the box and the truck
keeps the box from sliding as the
truck accelerates to the left as
shown.Which of the following
diagrams best describes
the static frictional force
acting on the box?
(A) (B) (C)
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Prelecture Question
Unit 6, Slide 22
What is the magnitude of the static frictional force
acting on the box?
A box of mass m sits at rest on an inclined
plane that makes and angle θ with the
horizontal. It is prevented from sliding by
static friction. The coefficient of static
friction between the box and the ramp is
μs.
A. μsmg
B. μsmg cosθ
C. mgsinθ
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Checkpoint
Mechanics Lecture 6, Slide 23
A box of mass M sits on a horizontal table. A horizontal string having tension T applies a force on the box, but static friction between the box and the table keeps the box from moving.
What is the magnitude of the static frictional force acting on the box?
A) Mg
B) mMg
C) TD) 0
M
f T
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Example 6.5 (Keeping a Block in Place)
The two blocks shown above are sliding across a frictionless surface by a force F from the left. The two blocks are not attached but the coefficient of static friction between the two is μs = 0.37. The mass of the smaller block is m1 = 19.0 kg and the mass of the larger block is m2 = 85.0 kg.
What minimum force F is needed to keep M1 from
falling down?
M2
M1F
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Question
Given the situation just described, which of the following best describs the freebody diagram of M2?.
(A) (B) (C) (D)
M2
M1F
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Question
Given the situation just described, which of the following best describs the freebody diagram of M1?.
(A) (B) (C) (D)
M2
M1F
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Question
Given the situation just described, which of the following best describs the freebody diagram of M2?.
(A) (B) (C) (D)
M2
M1F
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Question
Let’s say that instead of pushing on M2, M1 had been pushed against a fixed wall. What would the minimum value for the magnitude of F be that would hold M1 in place?
M1F
A) M1*g
B) g
C) 0
D) M1*g*ms
E) M1*g/ms
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Example 6.5 (Keeping a Block in Place)
The two blocks shown above are sliding across a frictionless surface by a force F from the left. The two blocks are not attached but the coefficient of static friction between the two is μs = 0.37. The mass of the smaller block is m1 = 19.0 kg and the mass of the larger block is m2 = 85.0 kg.
What minimum force F is needed to keep M1 from
falling down?
M2
M1F
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Example 6.5: Limiting Case Check
.
What minimum force F is needed to keep M1 from
falling down if M2 >>> M1?
M2
M1F
A) M1*g
B) M2*g*ms
C) (M1 + M2)*g*ms
D) M1*g*ms
E) M1*g/ms
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Example 6.5 (Keeping a Block in Place)
The two blocks shown above are sliding across a frictionless surface by a force F from the left. The two blocks are not attached but the coefficient of static friction between the two is μs = 0.37. The mass of the smaller block is m1 = 19.0 kg and the mass of the larger block is m2 = 85.0 kg.
What minimum force F is needed to keep M1 from
falling down?
M2
M1F
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Air Drag
|FDRAG|= bvn
Mechanics Lecture 6, Slide 32
Determined by shape
and cross-sectional
area.
Determined by range of velocity
- slow speeds n ~ 1
- most speeds we deal with n ~ 2
Terminal Velocity: When |FGRAV| = |FDRAG| for falling object
Example:
Air Drag on your car is 1.6 times greater at
70mph than at 55mph (70mph/55mph)2
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Mechanics Lecture 6, Slide 33
Question
A rider in a “barrel of fun” finds herself stuck
with her back to the wall. Which freebody
diagram correctly shows the forces acting on
her?
A B C D E
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Mechanics Lecture 6, Slide 34
Example 6.6 (Car on Curve)
A 2000kg car goes around
a corner with a radius of
60m at a speed of
20m/sec. The coefficient of
static friction between the
road and the cars tires is
0.8. Will it skid off the
road?
First thing: FBD
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Example 6.6: Question
Mechanics Lecture 6, Slide 35
A 2000kg car goes around a
corner with a radius of 60m at a
speed of 20m/sec. The
coefficient of static friction
between the road and the cars
tires is 0.8.
What is the force of friction if the car tries to go
around the curve at 20m/sec?
A) 0N
B) 13,330N
C) 15,680N
D) 20,830N
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Example 6.6: Question
Mechanics Lecture 6, Slide 36
A 2000kg car goes around a
corner with a radius of 60m at a
speed of 20m/sec. The
coefficient of static friction
between the road and the cars
tires is 0.8.
Will it skid off the road?
A) yes, it will skid off the road
B) no, it won’t skid
C) we can’t tell yet
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Mechanics Lecture 6, Slide 37
Example 6.6 (Car on Curve)
A 2000kg car goes around
a corner with a radius of
60m at a speed of
20m/sec. The coefficient of
static friction between the
road and the cars tires is
0.8. Will it skid off the
road?
First thing: FBDWhat is the car’s
speed is 25m/sec?
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Example 6.6: Question
Mechanics Lecture 6, Slide 38
A 2000kg car goes around a
corner with a radius of 60m at a
speed of 20m/sec. The
coefficient of static friction
between the road and the cars
tires is 0.8.
What is the force of friction if the car tries to go
around the curve at 25m/sec?
A) 0N
B) 13,330N
C) 15,680N
D) 20,830N
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Mechanics Lecture 6, Slide 40
Example 6.7 (Ferris Wheel)
Jessie is traveling a on ferris
wheel that has a radius of 4
meters. Her tangential velocity
is 3 m/sec and her mass is
55kg.
What is the normal force on
Jessie at the when she’s at the
top of the wheel?
What is the normal force on
Jessie at the when she’s at the
bottom of the wheel?
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Mechanics Lecture 6, Slide 41
Question
You are pushing a box across a
concrete floor with a force F1.
The box slides with a constant
velocity, v1.
If you wanted to slide the box
across the floor with a constant
velocity twice as large (v2 =
2*v1), how would this new force
compare to F1?A. F2 = 4*F1
B. F2 = 2*F1
C. F2 = 2*F1
D. F2 = F1
E. F2 = F1/2
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Mechanics Lecture 6, Slide 42
Question
You are pushing a box across a concrete
floor with a force F1. The box slides with
a constant velocity, v1. You now push on
the box with a force F2 which is twice
that of F1 (F2 = 2*F1).
The box will now
A. Move with a continuously increasing speed.
B. Move with a constant speed that is double that of v1
C. Move with the same constant speed of v1.D. Increase its speed for a while and then move
with a constant speed that is greater than v1.